The following abbreviations are herewith defined, at least some of which are referred to within the following description.
3GPP Third Generation Partnership Project
CCA Clear Channel Assessment
DL Downlink
ECCA Extended Clear Channel Assessment
eNB Evolved Node B
ETSI European Telecommunications Standards Institute
FBE Frame Based Equipment
FDMA Frequency Division Multiple Access
LAA Licensed Assisted Access
LBE Load Based Equipment
LBT Listen Before Talk
LTE Long Term Evolution
MCS Modulation and Coding Scheme
MU-MIMO Multi-User, Multiple-Input, Multiple-Output
OFDM Orthogonal Frequency Division Multiplexing
PCell Primary Cell
SC-FDMA Single Carrier Frequency Division Multiple Access
SCell Secondary Cell
UE User Entity/Equipment (Mobile Terminal)
UL Uplink
UMTS Universal Mobile Telecommunications System
WiMAX Worldwide Interoperability for Microwave Access
In Wireless Communications networks, for example, in LTE systems using LAA, an unlicensed spectrum is used with assistance from a licensed carrier. LAA may facilitate a fair coexistence with other technologies over the unlicensed spectrum and satisfy various regulatory requirements in different countries and regions.
For regulation requirements in Europe, ETSI has specified two channel access mechanism (i.e., FBE and LBE). For these two channel access mechanisms, before starting transmissions on an operating channel, the equipment (i.e., FBE and LBE) may perform a CCA check by using energy detection with the CCA observation time not less than 20 microseconds (“us”). If the energy level in the channel does not exceed a predefined threshold corresponding to the power level, the equipment may consider the operating channel to be clear and may transmit immediately. In contrast, the equipment may consider the operating channel to be occupied and may continue to perform the CCA check. For FBE, the equipment may continue to perform the CCA check at an end of a frame period. For LBE, the equipment may start performing ECCA immediately until it can grab the channel. In some situations, LBE may have a higher channel access probability than FBE.
Although LBE may have a higher channel access probability than FBE, FBE may be more appropriate for LAA UL. For example, FBE can follow the LTE UL framework that a UE's UL transmission should be permitted by a serving eNB, FBE can avoid inter-UE blocking and enable UL multiplexing of multiple UEs in one subframe by FDMA and MU-MILO. FBE does not require a reservation signal, and FBE has a fixed timing relationship and UL transmission can always start from the first OFDM symbol of a subframe. Accordingly, in certain configurations, FBE may be used as a baseline for LAA UL operation.
To support UL multiplexing of multiple UEs in one subframe by FDMA or MU-MIMO, LBT/CCA should be synchronous between UEs one the same carrier. Otherwise, the first UE would reserve the operating channel and the other UEs would see it as occupied, which would block multiplexing of multiple UEs in a UL subframe. For example, in a configuration in which a first (e.g. UE1) and a second UE (e.g., UE2) operate in an asynchronous manner, if UE1 performs CCA before UE2 and UE1 will transmit data immediately on the unlicensed spectrum if UE1 passes its CCA. Then, when UE2 performs CCA, it will find the channel is occupied and cannot transmit data unless UE1 stops transmission. This inter-UE blocking issue may not only block multi-user multiplexing in one subframe but also may inhibit fairly sharing the unlicensed spectrum.
To avoid the inter-UE blocking issue for FBE and enable multi-user multiplexing in one subframe, LBT/CCA may be synchronized among the UEs. Specifically, in one cell, all served UEs may be synchronous and scheduled from the first UL subframe in one frame period. For example, a frame period may include an occupancy time followed by an idle period. LBT/CCA for all served UEs in a cell may occur at the end of the idle period. Thus, a UE may transmit only depending on an eNB's scheduling, thereby using the LTE UL framework. Because all the served UEs perform CCA in a synchronous manner, UL transmission can start only at the beginning of each frame period.
There are two kinds of scheduling methods for LAA UL, i.e., cross-carrier scheduling and self-scheduling. Cross-carrier scheduling used a licensed carrier to schedule unlicensed secondary carriers and can avoid using the eNB to perform a CCA check before each UL grant transmission. In contrast to cross-carrier scheduling, self-scheduling requires the eNB to perform the CCA check before each UL grant transmission. Therefore, compared to self-scheduling, cross-carrier scheduling enable the eNB to always have the opportunity to transmit the UL grant transmission. However, when there are many secondary carriers on the unlicensed spectrum that need to be scheduled from a few licensed carriers (that is especially true for one PCell to schedule many SCells on a 5 GHz unlicensed spectrum) cross-carrier scheduling for unlicensed SCells may lead to DL control resource congestion (i.e., not enough control resources to transmit many UL grant transmissions). Accordingly, signaling overhead for UL grant transmissions may be excessive.